Manufacture of sodium fluoride



Patented Aug. 14, 1923.

UNITE HENRY HOWARD, 0F CLEVELAND, OHIO, ASEIGNOR TO THE GIELMQELLE@EZJEEIL'CAL COMPANY, OF CLEVELAND, 015101, A COEPUBLATMDN Gil @HEQ.

MANUFACTURE OF SODIUM FLUQEIDE.

Iio mawing.

To all whom it may (Jo 120mm,:

Be it known that I, HENRY HOWARD, a citizen of the United States,residing at Cleveland, in the county of (,uyahoga and State of Ohio,have invented certain new and useful Improvements in the Manufacture ofSodium Fluoride, of which the following is a specification.

This invention relates to a process primarily designed for themanufacture of sodium fluoride but also resulting in the production ofthe valuable by-products, artificial cryolite and ammonium chlorid.

The usual process of making sodium fluoride comprises neutralizinghydrofluoric acid with soda ash, sodium carbonate. This process issimple but since soda ash is comparatively expensive and no by-iproductsare formed to ofl-set the high cost of the soda ash, the sodium fluorideproduced is rather costly.

I have found that sodium fluoride can be prepared from sodium chlorid asa starting material instead of the more expensive soda ash. However, inorder to obtain economical yields of sodium fluoride by reacting uponsodium chlorid with hydrofluoric ac d it is necessary to neutralize thehydrochloric acid generated by the reaction in order that the reactionNaCl+HF-NaF may proceed to substantial completion.

My process therefore, consists in treating sodium chlorid withhydrofluoric acid and in neutralizing the hydrochloric acid as it isgenerated. As a neutralizing agent I use ammonia, preferably in the formof ammonical gas liquor.

According to my process I introduce hydrofluoric acid gas into asolution of sodium chlorid and simultaneously feed into the solutioneither gaseous or aqua ammonia, preferably gas liquor, at a rate justsuflicient to maintain the reaction mixture slightly acid untilhydrofluoric acid chemically equivalent to the sodium chlorid in thesolution has been absorbed, whereupon the supply of hydrofluoric acid isstopped and the reaction mixture is neutralized by further addition ofammonia. The solution of sodium chlorid and the solution of ammonia, incase aqua ammonia is employed, must be sufficiently dilute to avoid theprecipitation of ammonium chlorid.

The process is carried out ordinarily at room temperature, say about F.but the Application filed March 1, 1922. Serial No. 5%,303

reaction mixture becomes considerably warmed due to the heat generatedby the reaction. After the reaction is completed, as described, thereaction mixture is cooled to about 60 F. and filtered to separate theprecipitate of sodium fluoride which is then washed and dried and isready for use.

The wash waters are conveniently used for dissolving the next batch ofsodium chlorid.

The filtrate contains sodium fluoride, usually from 3 to 6 per cent, andammonium chlorid. This sodium fluoride may be separated and recoveredand the ammonium chlorid simultaneously purified by precipitat ng thesodium fluoride in the form of artificial cryolite. For this purpose itadd to the filtrate a solution of aluminum fluoride in quantitysuflicient to combine with the sodium fluoride present to form thecompound Al F .6NaF. The resulting precipitate of artificial cryolite isseparated from the l quid by filtration, washed and dried and isavailable for use. There is a good market for this product, for instancein the enameling industry.-

The filtrate separated from the artificial cryol te is a concentrated,substantially pure solution of ammonium chlorid from which ammoniumchlorid ready for the market may be preparedb concentrating the solutionand crysta izing the ammonium chlorid.

The purpose of treating the solution of sodium chlorid with hydrofluoricacid and ammonia simultaneously is to avoid substantial loss of either.Both ammonia and hydrofluoric acid are quite volatile, and if either isadded alone followed by the addition of the other some of the reagentwhich is added first and is therefore in large excess when the additionof the other rea cut is made, will be volatilized and lost. or instance,if the whole of the ammonia is added first, part of it will bevolatilized by the heat generated when the hydrofluoric acid is added,and a further amount of ammoiiia will be carried away by the inert gasesaccompanying the hydrofluoric acid gas.

A similar loss of hydrofluoric and hydrochloric acid will occur if thesolution of sodium-chlorid is first treated with hydrofluoric acid andthen with ammonia,'also the precipitated salt may consist in part of.sodium fluoride, artificial cryolite, and ammonium chlorid. are eachproduced in a comparatively pure state. The production I of pureammonium clilorid is a particularly advantageous feature. of the processsince the purification of this product results in the production ofanother valuable product, artificial cryolite. The starting materialsfor the process and particularly the sodium chlorid and the ammonia areinexpensive and due to this and the other economies of the process theproducts are inexpensive.

I claim:

1. Process of making sodium fluoride which comprises simultaneouslyadding hydrofluoric acid and ammonia to a solution of sodium chlorid- 2.Process of making sodium fluoride which comprises simultaneously addinghydrofluoric acid gas and aqua ammonia to a solution of sodium chlorid.

3. Process of making sodium fluoride which comprises passing a stream ofhydrofluoric acid gas into a solution of sodium chlorid andsimultaneously feeding ammoniacal gas liquor into the solution at a ratejust sufficient to maintain the reaction mixture slightly acid.

4. Process of making sodium fluoride and ammonium chlorid whichcomprises continuously dissolving hydrofluoric acid gas in a solution ofsodium chlorid, neutralizing hydrochloric acid as it is generated in thesolution by addition of ammonia, and recovering sodium fluoride andammonium chlorid from the reaction mixture.

5. Process of making sodium fluoride and ammonium chlorid and purifyingthe ammonium chlorid with the production of arti ficial cryolite whichcomprises, reacting upon sodium chlorid in aqueous solution withhydrofluoric acid, neutralizing hydrochloric acid as it is generated inthe solution by reaction with ammoniacal gas liquor, separating solidsodium fluoride from the reaction mixture, adding aluminum fluoride tothe resulting mother liquor, separating the artificial cryolite soformed from the reaction mixture, and recovering ammonium chlorid fromthe resulting separated liquid.

In testimony whereof I aflix my signature.'

HENRY HOlVARD.

